National Water-Quality Assessment (NAWQA) Program

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Groundwater Quality in Principal Aquifers

Frequently Asked Questions (FAQs)


About the study

What are Principal Aquifers?
Principal Aquifers are regionally extensive aquifers that supply most of the groundwater pumped across the Nation for drinking water, irrigation, and other uses.  The USGS has identified the Principal Aquifers in the Nation.

What is the purpose of the Principal Aquifer studies?
The purpose of the studies is to improve our understanding of how natural features and human activities affect groundwater quality.  Each of the regional assessments focuses on water-quality issues of concern within a particular aquifer, as well as addressing general issues.   By knowing where contaminants occur in groundwater, what factors control contaminant concentrations, and what kinds of changes in groundwater quality might be expected in the future, we can ensure the availability and quality of this vital natural resource in the future.

Which Principal Aquifers were studied?
The quality of groundwater was assessed for 41 of the Nation’s Principal Aquifers.  These aquifers provide about 90 percent of the groundwater pumped for public supply and more than half of the water supplied by private domestic wells nationally.

Why assess the quality of groundwater?
About 115 million people in the United States rely on groundwater for drinking water, and the need for high-quality drinking-water supplies becomes more urgent as our population grows.  Although groundwater is a safe, reliable source of drinking water for millions of people nationwide, the high concentrations of some chemical constituents that occur in some groundwater can pose potential human-health concerns. Some of these contaminants come from the rocks and sediments of the aquifers themselves, and others are chemicals that we use in agriculture, industry, and day-to-day life. When groundwater supplies are contaminated, millions of dollars can be required for treatment so that the supplies can be usable. Contaminants in groundwater can also affect the health of our streams and valuable coastal waters.

What is a contaminant?
In these circulars, a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in groundwater that is manmade or that impairs the use of water for its intended purpose. Impairment is determined by comparing a measured concentration to benchmarks or guidelines. By this definition, all manmade compounds, such as pesticides and volatile organic compounds, are contaminants because they do not occur naturally in groundwater. If a constituent with a geologic source, such as arsenic, occurs in drinking water at a concentration above its human-health benchmark, it also is considered a contaminant.

What constituents were analyzed for Principal Aquifer studies?
Dissolved concentrations of major ions, trace elements, nutrients, pesticides, volatile organic compounds (VOCs), and organic carbon were measured in most samples collected for the Principal Aquifers studies.  For samples from a subset of wells, radionuclides, groundwater age tracers, stable isotopes, and microorganisms also were measured.  Water-quality properties, such as pH and dissolved oxygen, were measured for all samples.  Ancillary information, such as well use, well depth, depth to water, and land use, were recorded for each well sampled.  About 1,300,000 chemical analyses were done for this study.

How many wells were sampled?
In all, one sample was collected from each of 6,620 wells—3,669 for aquifer studies (to assess the quality of water in the parts of aquifers used for drinking-water supply), 1,793 for agricultural land-use studies (to assess the quality of shallow groundwater beneath agricultural land), and 1,158 for urban land-use studies (to assess the quality of shallow groundwater beneath urban land).  Samples were collected primarily from public-supply wells, domestic-supply wells, and monitoring wells.

Was drinking water sampled?
Water samples were collected at the wellhead prior to any treatment. They represent the quality of the groundwater resource but not necessarily the quality of tap water.

Are there other national-scale assessments of groundwater quality?
The USGS has evaluated the quality of water from domestic wells and from public-supply wells in the U.S.  Information on those studies and associated reports can be accessed at and at


Human-health context for study findings

How were contaminant concentrations evaluated for human-health concerns?

To evaluate the potential significance of contaminant occurrence to human health, concentrations of contaminants that are regulated by USEPA in drinking water under the SDWA were compared to regulatory Maximum Contaminant Levels (MCLs), and concentrations of unregulated contaminants were compared to non-regulatory Health-Based Screening Levels (HBSLs), when available.  MCLs and HBSLs are collectively referred to as human-health benchmarks for these studies.  These comparisons provide an initial perspective on the potential significance of detected contaminants to human health and to help prioritize further investigations, but are not designed to evaluate specific effects of contaminants on human health, and are not a substitute for comprehensive risk assessments, which generally include many additional factors such as multiple avenues of exposure.

Concentrations greater than benchmarks are potential human-health concerns, but do not mean that adverse effects are certain to occur because the benchmarks are conservatively protective, and most samples were collected prior to any treatment or blending of water that potentially could alter contaminant concentrations.  Exposure to individual contaminants detected at concentrations less than benchmarks is unlikely to result in adverse human-health effects because the benchmarks typically are concentrations in drinking water that are not anticipated to cause adverse effects from a lifetime of exposure.

Are human-health benchmarks (MCLs and HBSLs) enforced?

MCLs are legally enforceable drinking-water standards that apply to finished drinking water from public water systems.  However, for the Principal Aquifer studies, contaminant concentrations greater than MCLs do not represent MCL violations because MCLs apply only to finished water—samples for the studies were collected from source water (that is, prior to treatment).  None of the source-water samples were collected for regulatory compliance purposes; further, compliance with most MCLs is based on running average concentrations, not on concentrations detected in single samples, as collected for these studies.  HBSLs are not enforceable drinking water standards.

Do water utilities generally treat water to remove the contaminants analyzed for this study from public supplies?

If contaminant concentrations are greater than MCLs, public water systems typically treat or blend source water with higher-quality water sources to decrease concentrations to less than MCLs.  Water utilities, however, are not required to treat water for unregulated contaminants (that is, those without MCLs).  HBSLs are not enforceable drinking water standards.
For additional information on public-supply well water quality:

Do homeowners generally treat water to remove the contaminants analyzed for this study from domestic (privately owned) water supplies?

Routine testing of water from domestic wells is not required, and homeowners are responsible for maintaining and monitoring wells and for obtaining treatment to resolve any water-quality problems.
For additional information on domestic well water quality:


Water quality in Principal Aquifers

How frequently did a sample from a drinking-water well contain a contaminant at a concentration that exceeded its human-health benchmark?

Groundwater from 22 percent of sampled wells—more than one in five—contained at least one contaminant at a concentration greater than a U.S. Environmental Protection (USEPA) Agency Maximum Contaminant Level (MCL) or other human-health benchmark for concentrations in drinking water.

Which contaminants most frequently exceeded a human-health benchmark?

Most of the contaminants that exceeded a benchmark were from geologic sources—for example, arsenic, manganese, radon, and uranium. Nitrate was the only constituent from manmade sources that exceeded its human-health benchmark in more than 1 percent of wells. 

What is meant by a contaminant “with a geologic source”?

Contaminants with geologic sources are those dissolved from the rocks and sediments that make up the aquifer.  Concentrations of contaminants with geologic sources tend to be higher in deeper, older groundwater because it has been in contact with the aquifer rocks and sediments for a long time.  Contaminants with geologic sources include arsenic, uranium, radon, and manganese.

What is meant by a contaminant “with a manmade source” or “from human activities”?

Contaminants with manmade sources or from human activities are those that have been synthesized for human use or whose concentrations in water are much greater than would occur naturally because of human activities.  Most pesticides and volatile organic compounds (VOCs), for example, are organic chemicals that have been synthesized for human use.  Nitrate, although it has natural sources, is assumed to be from human activities, such as fertilizer applications and wastewater disposal, if it occurs at concentrations above baseline (generally greater than 1 mg/L).


More information on these studies

How can I retrieve data used in the reports?

The index page for each circular includes a link to the data used in the report.  Links to index pages are available online by hovering over the Principal Aquifer of interest on the map. The data set used for the National Assessment of Groundwater quality is available at:

Where can I learn more about other NAWQA water-quality assessments?

Additional information about the USGS NAWQA Program is available online.
Learn more about the movement of contaminants in groundwater and in public-supply wells at:
The USGS Groundwater Age Mixtures and Contaminant Trends Tool is available online to explore the effects of basic aquifer properties and well configurations on groundwater age mixtures in groundwater discharge and on contaminant trends from varying nonpoint-source contaminant input scenarios.

What are the NAWQA monitoring and modeling priorities for the next decade?

Over the next decade, the National Water-Quality Assessment Program will continue to address the three central questions: 1.) what is the quality of the Nation’s groundwater, 2.) is it getting better or worse, and 3.) what factors are affecting the quality of this vital resource. About 2,300 shallow wells and 1,400 deep public-supply wells will be sampled for a broad range of water-quality constituents. National and regional scale modeling will provide a three-dimensional perspective of the quality of the Nation’s groundwater that can be used to inform management decisions.


Abbreviations and units of measure used in this FAQ
USGS – U.S. Geological Survey
USEPA – U.S. Environmental Protection Agency
SDWA – Safe Drinking Water Act
MCL – Maximum Contaminant Level
HBSL – Health-based screening level
mg/L – milligrams per liter

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Page Last Modified: Tuesday, 20-Jan-2015 12:01:55 EST